The present invention relates to optical signal distributors for use with an instrumentation system of an optical transmission type, and more particularly to an optical signal distributor for use with a field instrumentation system of an optical transmission type in which information from an plurality of field instruments installed on the field side is sent through an optical fiber transmission path and an optical signal distributor to a processor in a centralized control room.
Generally, a field instrumentation system applied to a chemical plant, a petroleum or a gas plant is comprised of a plurality of field instruments (for example, including pressure, flow and temperature measuring instruments and actuators such as openable/closable valves) installed in the field (spot) to send data between those field instruments and the central processing unit (controller) installed in the centralized supervision room or between those field instruments to supervise and control the respective plants. In order to send data in the system, optical fibers which (1) are not greatly influenced by electromagnetic noise, (2) exhibit high resistance to thunder (high insulation) and (3) are free from a danger of explosion are used as transmission paths, and an optical star coupler (especially, of a reflective type) is used as an optical signal distributor to send data (optical signal) between the central processing unit and each of the field instruments and between the field instruments.
FIG. 10 shows the structure of a system which uses a reflective type star coupler employing a mixing rod as an example of a conventional field instrumentation system. Reference numeral 90 denotes a controller; 97 a reflective type optical star coupler (optical signal distributor); 92 (92a-n) a field instrument; 93 and 94 (94a-94n) an optical fiber. Data transmission is possible between the controller and each of the field instruments and between the field instruments, using the reflective type optical signal distributor, various control operations are performed at high speed in a decentralized processing manner, and the system is fabricated at a low cost.
The reflective type optical signal distributor using the mixing rod will be described next in detail.
FIG. 11A is a perspective view of an example of the conventional reflective type optical signal distributor using the mixing rod.
The distributor 97 includes a flat mixing rod 96 which has thereto connected a plurality of optical fibers 8 at one end and a reflective plate 4 at the other end. Light entering the mixing rod 96 through any one of the optical fibers 8 is reflected by the reflective plate 4 and distributed to all the optical fibers 8. The mixing rod 96 has the functions of mixing the propagation modes of the incident light and reflected light from the reflective plate 4, and smoothing an optical intensity distribution at a cross section thereof, in other words, the function of equalizing an optical intensity distribution at a cross section of the mixing rod 96 to the utmost.
However, this conventional distributor has the defect that it will produce a large loss, which will next be described with reference to FIG. 11B which is a cross sectional view schematically indicative of connection of the mixing rod and the optical fibers.
In the reflective type optical signal distributor using the mixing rod, optical fiber claddings 73 of the optical fibers 8 are arranged so as to contact one to the other. The size of the cross section of the mixing rod 96 is designed so as to cover all the cores 72 of the optical fibers 8 to minimize a connection loss between the optical fibers 8 and the mixing rod 96. However, even when the connection loss is minimized so, most of light emitting from the fiber cores 72 enters the mixing rod 96, as shown in FIG. 11B, whereas a portion of the light exiting from the mixing rod 96 to the optical fibers 8 and impinging on the fiber claddings 73 and entering fiber spacings 98 are leaking components. Thus, such distributor necessarily has the defect that it will produce a large loss.
The mixing rod is not necessarily be flat, as described above, but may, for example, have a circular cross section. Even if the mixing rod may take any cross-sectional configuration, it is difficult to avoid leakage of light from the spacings between the optical fibers and from the fiber claddings.
Since in the optical transmission using the optical fibers, the optical fibers themselves produce an optical loss proportional directly to their length, their transmittable length is limited. Thus, when the distributor produces a large loss, the usable length of the optical fibers is shortened accordingly and the distance between field instruments connectable to the optical signal distributor is reduced. The conventional reflective type optical signal distributor using the mixing rod produces large leaking light and, hence, a large loss, so that its range of applications is limited.
It is therefore an object of the present invention to provide an inexpensive, low-loss, high branching accuracy, optical signal distributor for use in a field instrumentation system, the distributor (1) having an optical performance ever stabilized even when the temperature and humidity of its environment may change, (2) having an optical performance which is not or less deteriorated in an environment which contains a large amount of dust, organic substances, and/or corrosive gases, (3) having high environment resistance characteristic, (4) having high resistance to mechanical stresses such as vibrations and/or impacts, and (5) being easy to place on the spot.